LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 37 



FIG. 27. Diagram showing fundamental structure of types of several animal phyla: 

 1, sea anemone; 2, starfish; 3, worm; 4, centipede; 5, clam; 6, honeybee; 7, sala- 

 mander. In each figure the central nervous system is indicated by the black lines. 

 (After Haeckel.) 



But great periods of this time are shut away from us without 

 record of their duration, and long series of the gradually 

 changing forms are lost to us without hope of discovery. And 

 yet in its large outlines we know the history of all this time 

 and the character of all these graded series. 



